Improving LED (Light Emitting Diode) light efficiency is a top priority for realizing a solid-state illuminator. There are two approaches to improve LED light efficiency: one is to increase LED internal quantum efficiency, and the other is to increase LED external quantum efficiency (LED light extraction efficiency).
There is a great difference between the refractive indexes of a semiconductor and a packaging material; therefore, a LED usually has a small total reflection critical angle. The light generated by LED reaching an interface by an angle greater than the total reflection critical angle will be totally reflected back to the interior of the LED chip. Then, the probability that light leaves a semiconductor from an interface decreases, and photons can only be totally reflected inside a chip until they are completely absorbed and converted into heat. Thus, LED has an insufficient light efficiency.
Changing the geometry of LED is an effective measure to improve LED light efficiency. A U.S. Pat. No. 7,075,115 disclosed a semiconductor light emitting element, which has a concave and convex structure formed on the border of the LED element. Compared with the planar surface of another LED element, the concave and convex structure can scatter or diffract the horizontally-propagating light and thus can greatly promote external quantum efficiency.
In the abovementioned conventional technology, the fabrication process of the concave and convex structure includes: forming a passivation layer on the surface of the semiconductor layer of the LED; patterning the geometry of the concave and convex structure on the passivation layer with a photolithographic process; and etching the semiconductor layer of the LED to form the concave and convex structure with a dry- or wet-etching method. However, such a process is more complicated and cost-inefficient, which will impair the commercialization of LED.